It is generally known and an accepted commercial practice to add a blowing agent to various polymeric materials during fabrication such as to produce a cellular (expanded foam) material. Typically, the blowing agent can be either a reactive solid or liquid that evolves a gas, a liquid that vaporizes, or a compressed gas that expands during final fabrication producing the desired polymeric foam. Such foams are categorically either closed cell (i.e., non-porous, continuous polymer phase with discontinuous gas phase dispersed therein) or open cell (porous) foams which are advantageously employed in various end use applications and exhibit various advantages associated with the particular type of foam produced. In describing the closed cell foam as involving a discontinuous gas phase, it should be appreciated that this description is an over simplification. In reality the gas phase is dissolved in the polymer phase and there will be a finite substantial presence of gas (blowing agent) in the polymer. Furthermore and as generally known in the art, the cell gas composition of the foam at the moment of manufacture does not necessarily correspond to the equilibrium gas composition after aging or sustained use. Thus, the gas in a closed cell foam frequently exhibits compositional changes as the foam ages leading to such known phenomenon as increase in thermal conductivity or loss of insulation value.
Closed cell foams are usually employed for their reduced thermal conductivity or improved thermal insulation properties. Historically, insulating polyurethane and polyisocyanurate foams have been made using trichlorofluoromethane, CCl.sub.3 F (CFC-11), as the blowing agent. Similarly, insulating phenolic foam is known to be made from phenol-formaldehyde resins (typically via an intermediate resole mixture involving a phenol-formaldehyde oligomer condensate) using blends of 1,1,2-trichlorotrifluoroethane, CCl.sub.2 FCClF.sub.2 (CFC-113), and CFC-11 as the blowing agent. Also, insulating thermoplastic foam such as polystyrene foam is commonly manufactured using dichlorodifluoromethane, CCl.sub.2 F.sub.2 (CFC-12), as the blowing agent.
The use of a chlorofluorocarbon as the preferred commercial expansion or blowing agent in insulating foam applications is in part based on the resulting k-factor (i.e., the rate of transfer of heat energy by conduction through one square foot of one inch thick homogenous material in one hour where there is a difference of one degree Fahrenheit perpendicularly across the two surfaces of the material) associated with the foam produced. Thus, it is generally known and accepted that a chlorofluorocarbon gaseous phase within the closed cell is a superior thermal barrier relative to other inexpensive gases such as air or carbon dioxide. Conversely, the natural intrusion of air into the foam over time and to a lesser extent the escape of the chlorofluorocarbon from the cell is deleterious to the desired low thermal conductivity and high insulative value of the foams. Also, the escape of certain chlorofluorocarbons to the atmosphere is now recognized as potentially contributing to the depletion of the stratospheric ozone layer and contributing to the global warming phenomenon. In view of the environmental concerns with respect to the presently used chlorofluorocarbon blowing agents, it is now generally accepted that it would be more desirable to use hydrochlorofluorocarbons or hydrofluorocarbons rather than the chlorofluorocarbons. Consequently, the need for a method or way of inhibiting the permeation of air and blowing agent through the polymer phase of the polymeric foam exists and hopefully any such solution to the problem would be effective in inhibiting the permeation of the proposed alternative halocarbons.
Historically, various methods and compositions have been proposed, with varying degree of success, to alleviate and/or control problems associated with permeation of gases into and out of polymeric foams. For example, in U.S. Pat. No. 4,663,361 the problem of shrinkage (lack of dimensional stability) associated with using any blowing agent other than 1,2-dichlorotetrafluoroethane in the manufacture of foamed polyethylene is addressed. In this reference, a stability control agent is used in either a homopolymer or copolymer of ethylene wherein the blowing agent is isobutane or isobutane mixed with another hydrocarbon or a chlorocarbon, fluorocarbon or chlorofluorocarbon. The stability control agent is either partial esters of long chain fatty acids with polyols, higher alkyl amines, fatty acid amides, olefinically unsaturated carboxylic acid copolymers, or polystyrene. This reference also describes other prior art and is included by reference for such purpose.
In U.S. Pat. No. 4,243,717 a Fischer-Tropsch wax is added to expanded polystyrene beads to produce a stable cell structure in the foam, without specific reference to the permeation of blowing agent or air. In Canadian Patent 990,900 the use of a barrier material or blocking agent is disclosed to alleviate the problem of gas migration through the cell wall specifically at the time of foaming. The particular problem addressed in this Canadian patent is the rupture and total collapse of the cell walls that frequently occur in the manufacture of closed cell polyethylene foam. This problem is attributed to the fact that the cell walls for such foams are permeable to the rapidly expanding gas under the influence of the heat liberated by the exothermic polymer crystallization. The specific solution disclosed in this reference is to use a blend of polyethylene and polypropylene along with a barrier resin such as a elastomer containing polystyrene or acrylic resin which are intended to contribute high melt strength to the cell wall at the foaming temperature. An inert nucleant is also employed along with at least two gaseous propellants of substantially different vapor pressures.
In U.S. Pat. No. 4,795,763 the use of at least 2 percent carbon black as a filler uniformly dispersed in a polymeric foam is shown to reduce the aged k-factor of the foam to below the aged k-factor of the corresponding unfilled foam.
In general, thermoplastic foams are manufactured by mixing a volatile blowing agent with a molten thermoplastic resin under controlled conditions of temperature and pressure sufficient to form a plasticized, resin-blowing agent mixture and maintain the mixture in an unfoamed state. Thereafter, the mixture is extruded through a suitable die into a zone of lower pressure at a controlled temperature to obtain a substantially closed cell structure having a desired shape and form.
For satisfactory production of closed cell foams, the blowing agent's solubility should be sufficiently high, while the mixture is passing through the die into the lower pressure expansion zone, so that expansion proceeds smoothly to the closed cell state. Otherwise, if the solubility is too low the blowing agent vaporizes away prematurely--and so rapidly and to such an extent before the cell walls fully form--that the resulting foam contains a high proportion of ruptured cells (voids), which adversely affects the foam's utility, e.g., as insulating or load-bearing structural material. The blowing agents solubility in the resin is particularly critical when extruding through large cross-sectional dies. This is because the throughput of the extruding mixture is generally fixed in conventionally employed extruders and the back-pressure at the lip (opening) is low. The larger the area of the die opening the lower is the back-pressure exerted on the mixture and the greater is the number of voids in the resulting foam.
There are many conventional methods for making thermoplastic foams. Siraux et al, European Patent Application No. 0 406 206 A2 discloses polystyrenic foams and a method of making them involving a mixture of blowing agents comprising dichloromethane and one or more of a hydrochlorofluorocarbon (HCFC), a hydrofluorocarbon (HFC) and a fluorocarbon (FC), with dichloromethane comprising 5 to 25 weight percent of the mixture. The disclosed system has the disadvantage of utilizing and requiring environmentally objectionable (e.g., ozone depleting) chloro-containing volatile substances as part of the blowing agent composition.
Suh et al, Canadian Patent No. 1,086,450, relates to polystyrenic closed cell insulating foams and their preparation, utilize a mixture of high permeability and low permeability blowing agents for foam production. These foams are undesirable because they require high proportions of objectionable flammable and/or chloro-containing high permeability blowing agents.
Suh, U.S. Pat. No. 5,011,866, discloses blowing agents comprising at least 70 weight percent HFC-143a (1,1,1-trifluoroethane) or HFC-134a (1,1,1,2-tetrafluoroethane) for the preparation of polystyrenic insulating foams having small closed cells, low densities (1 to 6 pounds per cubic foot) and high dimensional stabilities among other properties. HFC-143a is objectionable for its flammability, and HFC-134a presents processing difficulties as indicated in York U.S. Pat. Nos. 5,146,896 and 5,204,169.
Omire et al, U.S. Pat. No. 5,145,606, disclose eleven categories of mixed blowing agents for foaming thermoplastics such as polystyrene and polyethylene. Included among the blowing agent mixtures are four that comprise a tetrafluoroethane, which may be HFC-134a and/or HFC-134 (1,1,2,2-tetrafluoroethane), mixed with one or more selected hydrochlorofluorocarbons (HCFC). Omire, however, teaches that HFC-134 and HFC-134a cannot be advantageously used alone as a blowing agent for thermoplastics, and tetrafluoroethane should be used in combination with at least one chloro-containing blowing agent.
Rubin et al, U.S. Pat. No. 5,314,926, describe blowing agents for polystyrene and other non-isocyanurate based foamable plastics comprising 1,1,1,2,3,3,3-heptafluoropropane (HFC-227ea) in combination with one or more hydrocarbons or partially halogenated alkanes, which may contain chlorine substituents or be chlorine free. The disclosed system has the disadvantage of requiring HFC-227ea, a high HGWP (0.6) material, i.e. environmentally objectionable, as part of the composition.
Volcker et al, U.S. Pat. No. 5,334,337, disclose high compression strength foam board prepared from polystyrene containing 5-16 weight percent of a blowing agent mixture containing an alcohol or ketone, carbon dioxide (CO.sub.2) a C.sub.3 -C.sub.5 hydrocarbon and fluorinated hydrocarbons. The disclosed technology is disadvantageous in that it includes at least one flammable component which is also a volatile organic compound (VOC); the usage of which is regulated in many countries.
Bartlett et al, U.S. Pat. No. 5,182,040, disclose azeotropic and azeotropic-like binary compositions of HFC-134 with HFC-152a (1,1-difluoroethane), dimethyl ether (DME) and selected halocarbons and hydrocarbons are useful as refrigerants, aerosol propellants and blowing agents for polymer foams.